Many optical systems require precision optomechanical mountings that hold optical elements in the positions and orientations required for operation of the systems. To achieve proper positioning and alignment of an optical element, an optomechanical mounting generally must allow movement or rotation of the optical element relative to other optical elements during an alignment process, but once the optical element is aligned the mounting must securely hold the optical element to maintain the proper alignment during shipping and use of the optical system. A conflict exists between the need to align an optical element with great precision and the need to have the optical element remain aligned for the lifetime of the optical system.
To provide adjustability for alignment and still securely hold the optical element in position, optomechanical mounts often use clamping systems. In particular, an optical element in the optomechanical mounting can be adjusted or aligned when the clamp is loose, but the optical element is rigidly held when the clamp is tightened. One concern in optomechanical mounts using clamps is disturbance of the optical element""s alignment when clamping the optomechanical mount.
Using the same clamping friction during alignment and use of an optical system avoids disturbances that arise from post-alignment clamping but requires a tradeoff between alignment precision and operational stability. In particular, a multi-axis interferometer typically requires optomechanical mounts that can precisely orient laser beams with sub-microradian sensitivity. An optomechanical mount with a pure kinematic design can provide sub-microradian sensitivity, but typically cannot hold that alignment when subjected to shock, vibrations, and temperature changes. A ruggedly clamped, and therefore stable, optomechanical mount generally is difficult to adjust with sub-microradian precision. Accordingly, tradeoffs are generally required between the precision of adjustments and the stability, and a semikinematic design is often the compromise.
FIG. 1 shows an optomechanical mounting 100 that emphasizes minimal constraint and adjustment sensitivity. Such optomechanical mountings are further described in U.S. Pat. No. 6,170,795. Optomechanical mounting 100 includes a support 14, a three-sphere nest 20, a sphere 12, a top plate 14, and a spring preloaded plunger 26 (spring not shown) operated by a clamp screw 28. Sphere 12 contains an optical element such as a mirror (not shown) that can be rotated on three-sphere nest 20 for an alignment process that changes the orientation of the optical element without changing the position of its optical center.
Alignment of the optical element, which is fixed at the center of sphere 12, generally requires loosening clamp screw 28 to relieve or reduce the clamping force on sphere 12 and permit rotation of sphere 12. The spring (not shown) bearing on plunger 26 thus applies an initial stabilizing force directed along a radius of sphere 12. Once sphere 12 is aligned, tightening clamp screw 28 overcomes the spring and causes plunger 26 to apply the clamping force, which holds sphere 12 in the proper orientation. The final clamping force is set by applying a prescribed torque to clamp screw 28, and the frictional forces resulting from the clamping force resist rotation of sphere 12 to retain the alignment of the optical element.
The need to loosen clamp screw 28 before alignment and the need to tighten clamp screw 28 after alignment increase the total time required for the alignment process. Additionally, clamp screw 28 may not be easily accessible in an optical system, which makes the alignment process more difficult. Additionally, tightening clamp screw 28 causes bending of the assembly and hence disturbs the accuracy of the just-completed alignment. For most applications, an optomechanical mounting is desired that maintains precise angular orientation of the optical element without requiring additional procedures to apply a clamping force after the alignment process.
In accordance with an aspect of the invention, an optomechanical mounting includes an upper spring assembly and a lower spring assembly that support and secure a sphere containing an optical element fixed at its center. The spring assemblies can be substantially identical so that thermal expansions affecting one assembly compensates for identical opposing thermal expansion affecting the other spring assembly, giving the optomechanical mounting superior thermal stability. Frictional forces on the sphere from the upper and lower spring assemblies maintain the orientation of the sphere (and the optical element in the sphere) during operation but still permit rotation of the sphere for alignment without removing either spring assembly or releasing the spring tension that the spring assemblies apply to the sphere.
Each spring assembly can include springs around a perimeter of a ring so that a central region of each assembly is open for an optical path through the assembly. Alternatively, the central region can be open to provide access for tools that facilitate rotation of the sphere for alignment of the optical element.
Embodiments of the invention can exhibit excellent long-term alignment stability when subjected to temperature changes, shock, and vibration. The symmetry of the optomechanical mount and the use of similar construction materials in the elements of the mount provide excellent thermal stability. High clamping forces between the springs and the sphere resist alignment changes caused by mechanical shock. In particular, frictional forces at multiple points on the sphere resist rotation of the sphere after alignment is achieved, but fine surface finishes on the sphere and spring make smooth, high resolution rotational adjustment achievable with removable alignment tools. Vibration stability results because the springs, which stiffen due to geometrical deformation as a result of high compressive forces, wrap tightly around the sphere to provide a stiff, highly damped spring/mass system having a high resonant frequency, typically greater than 3 kHz.
One specific embodiment of the invention is a system that includes a sphere adapted for mounting an optical element, a first set of springs including multiple springs in contact with the sphere; and a second set of springs including multiple springs in contact with the sphere. Generally, each spring in the first set has a corresponding spring in the second set, and each spring in the first set applies a force to the sphere that is collinear with and opposite to a force that the corresponding spring in the second set applies to the sphere. All spring forces are directed through the center of the sphere. The opposing forces from the springs maintain positional stability of the sphere when the optomechanical mounting is subjected to thermal variations, vibrations, or shock.
Typically, either set of springs can be mounted on an inner surface of a support ring. The inner surface of the support ring is typically a conic section with fixtures for mounting the springs, and each spring can be a leaf spring set at an angle according to the normal to the sphere""s surface where the spring contacts the sphere. The support rings can be open in the center for light paths of the optical element or for access to the sphere during the alignment process. A case on which the support rings are mounted can control the separation of the first and second set of springs or their associated support rings to control magnitudes of forces that the first and second sets of springs apply to the sphere.